1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member. In particular, the present invention relates to an electrophotographic photosensitive member optimum for a printer, a facsimile, a copying machine, or the like using light having a relatively short wavelength of 380 nm or more and 500 nm or less for exposure.
2. Related Background Art
In the field of image formation, a photoconductive material in a photosensitive member is requested to have properties including the following properties:
1. High sensitivity and a high SN ratio (photo current (Ip)/dark current (Id))
2. An absorption spectrum suited for the spectral characteristics of an electromagnetic wave to be applied
3. High photoresponsiveness and a desired dark resistance value
4. Harmlessness to a human body at the time of use
In particular, it is important for an electrophotographic photosensitive member to be incorporated into an electrophotographic device to be used as a business machine in an office to be pollution-free at the time of use.
Amorphous silicon (hereinafter, abbreviated as a-Si) is a photoconductive material exhibiting excellent properties satisfying the above-described properties, and has been attracting attention as a photoreceptive member of an electrophotographic photosensitive member.
A photosensitive member having a photoconductive layer composed of a-Si is generally formed on a conductive substrate heated to 50° C. to 350° C., by a film forming method such as a vacuum deposition method, a sputtering method, an ion plating method, a thermal CVD method, a photo CVD method, or a plasma CVD method. Of those methods, a plasma CVD method has been suitably put into practical use, involving: decomposing a raw material gas by means of a high-frequency wave or through microwave glow discharge; and forming an a-Si deposition film on a substrate.
For example, Japanese Patent Application Laid-open No. H05-150532 or the like discloses an a-Si photosensitive member composed of a substrate, a barrier layer, a photoconductive layer, and a surface protective layer. The photosensitive member is brought into the reverse bias state of a p-i-n junction by: using SiH4, H2, N2, and B2H6 as raw material gases; and specifying the flow rate ratio of each raw material gas.
In addition, Japanese Patent Application Laid-open No. H08-171220 discloses an electrophotographic photosensitive member including: a conductive substrate; and a photoconductive layer composed of a-Si and a surface layer composed of amorphous silicon nitride on the substrate, in which the outermost surface of the electrophotographic photosensitive member has an element composition ratio N/Si in the range of 0.8 to 1.33 and an element composition ratio O/Si in the range of 0 to 0.9.
In addition, in order to satisfy the recent demand for an increase in image quality, the high definition of an electrostatic latent image as well as a reduction in particle size of toner has been increasingly requested. One possible method of satisfying the demand in, for example, a digital copying system is a method involving reducing the spot diameter of laser to be used for image exposure. To this end, a reduction in wavelength of laser has been requested. Japanese Patent Application Laid-open No. 2000-258938 proposes an image forming apparatus in which a photosensitive layer is formed of an a-Si hydride and which uses an ultraviolet violaceous laser beam oscillator having, as an exposure wavelength, a predominant oscillation wavelength at 380 nm to 450 nm.
Furthermore, Japanese Patent Application Laid-open No. 2002-311693 proposes an electrophotographic device using an a-Si-based photosensitive member in which: an electric field to be applied to the photosensitive member upon exposure with an image forming light beam is 150 kV/cm or more; and the image forming light beam has a wavelength of 500 nm or less.
Examples of a method of charging an a-Si photosensitive member include: a corona charging system in which corona discharge is employed; a roller charging system in which charging is carried out through direct discharge by the use of a conductive roller; and an injection charging system in which a contact area is sufficiently extended by means of magnetic particles or the like and charging is carried out through the direct injection of charges into the surface of a photosensitive member.
Of those, in each of the corona charging system and the roller charging system, a corona product is apt to adhere to the surface of a photosensitive member because each of the systems employs discharge. In addition, an a-Si photosensitive member has a surface layer having a hardness much higher than that of an organic photosensitive member or the like, so the surface layer is difficult to abrade and a corona product is apt to remain on the surface. In that case, the corona product and water bind to each other owing to moisture adsorption of water in a high-humidity environment or the like to reduce the electrical resistance of the surface, hence the charges of the surface is apt to move. As a result, an image deletion phenomenon may occur. To cope with the foregoing, various devices such as a method of rubbing the surface and a method of managing the temperature of the photosensitive member are required in some cases.
In contrast, the injection charging system is a charging system which does not actively employ discharge and involves directly injecting charges from a portion in contact with the surface of a photosensitive member. Therefore, it is difficult for a phenomenon such as the above-described image deletion to occur.
In addition, the injection charging system as contact charging is of a voltage control type while the corona charging system is of a current control type. Therefore, the injection charging system is advantageous in the respect that the unevenness of a charging potential is relatively easy to reduce.
The properties of a conventional a-Si-based electrophotographic photosensitive member such as electrical, optical, and photoconductive properties (such as a dark resistance value, photosensitivity, and photoresponsiveness), service environment properties, stability over time, and durability have been improved. However, at present, there is room for the conventional a-Si-based electrophotographic photosensitive member to be improved for enhancing comprehensive properties.
In particular, in recent years, digitalization and colorization have been rapidly promoted. As a result, there has been a growing demand for an increase in image quality of an electrophotographic device (such as high resolution, high definition, the absence of density unevenness, or the absence of image defects (for example, a void or a black spot)).
In a digital full-color copying machine, negative toner as color toner is used in combination with an image exposure method (a method involving exposing an image portion) having high controllability of a latent image and suited for an increase in image quality and a photosensitive member to be negatively charged. In an a-Si-based photosensitive member for negative charging, the improvement of the properties depends on how well the function of inhibiting the inflow of charges from the surface as much as possible works.
In addition, demands for an increase in speed and an increase in durability have been growing rapidly. Therefore, an electrophotographic photosensitive member is requested to improve in electrical properties, photoconductive properties, and uniformity, to reduce image defects, and to significantly improve in performance such as durability or environmental resistance (adaptability to a change in temperature or humidity).
Not only toner having a small particle size but also a reduction in the spot diameter of laser light for image formation is effective in increasing the resolution of an image. Examples of a method of reducing the spot diameter of laser light include an increase in the accuracy of an optical system for irradiating a photoconductive layer with laser light and an increase in the opening ratio of an imaging lens. However, the spot diameter can be reduced only up to a diffraction limit determined by the wavelength of laser light and the opening ratio of the imaging lens. Therefore, for reducing a spot diameter with the wavelength of laser light kept constant, an increase in the size of a lens, an improvement in machine accuracy, or the like must be performed, so increases in the size and cost of an apparatus are hardly avoided.
In view of the foregoing, in recent years, a technique for increasing the resolution of an electrostatic latent image in which the wavelength of laser light is shortened to reduce the spot diameter has been attracting attention, based on the fact that the lower limit of the spot diameter of laser light is in direct proportion to the wavelength of the laser light.
In a conventional electrophotographic device, laser light having an oscillation wavelength of 600 to 800 nm is generally used in image exposure. By shortening the wavelength, the resolution of an image can be increased. In recent years, a semiconductor laser having a short oscillation wavelength has been rapidly developed, and a semiconductor laser having an oscillation wavelength at around 400 nm has been put into practical use.
An a-Si photosensitive member in which resolution is as high as 2,400 dpi and a semiconductor laser having an oscillation wavelength of about 400 nm is used for image exposure has been desired.
In addition, toner having a small particle size to be used for a high-resolution digital full-color copying machine tends to present problems such as a transfer residue on the surface of a photosensitive member and a cleaning residue. Improvements to cope with the problems have also been requested.
In order for the resolution of an image to be increased by means of any one of the approaches exemplified above, a material for, in particular, the surface region of a photosensitive member has been requested to be further improved so that light having a wavelength in a short wavelength range around 400 nm can be applied to the photosensitive member.
For example, an a-Si-based photosensitive layer has a peak sensitivity at around 600 to 700 nm. The photosensitive layer can have a sensitivity even at around 400 to 410 nm under devised conditions, although the sensitivity is inferior to the peak sensitivity. Therefore, for example, the photosensitive layer can be used even when laser having a wavelength as short as 405 nm is used. However, the sensitivity at around 400 to 410 nm may be about half the peak sensitivity, so it is preferable that almost no absorption of light is present in the surface region of a photosensitive member.
However, with an amorphous silicon carbide (hereinafter, a-SiC) based material or an amorphous carbon (hereinafter, a-C) based material that has been suitably used for a surface layer heretofore, the absorption tends to be large at around 400 to 410 nm. In the case of the a-SiC-based material, the tendency can coped with through an increase in transmittance under devised conditions or a reduction in thickness of the surface layer to some degree. However, the surface layer is gradually abraded inevitably owing to rubbing in a copying machine, so the layer must secure at least a certain thickness. Accordingly, an increase in absorbed amount due to an increase in thickness and sensitivity unevenness due to abrasion unevenness may present a problem in some cases when high-resolution images are to be stably outputted.
In contrast, a film having good transmittance can be formed of the a-C-based material under some conditions. In this case, however, the film has a structure close to that of a polymer, and its hardness may be low or its resistance value may be too high. Therefore, the a-C-based material may establish a trade-off relationship between transmittance and hardness or resistance.
It has been found that an amorphous silicon nitride (hereinafter, a-SiN) based material can be used instead of those materials. However, a film made of such material can be hardly used for the surface layer of a photosensitive member, and it has not been put into practical use yet. For example, Japanese Patent Application Laid-open No. H08-171220 shows that various advantages and disadvantages appear depending on a raw material gas of a-SiN. The document shows that a specific condition must be selected for obtaining suitable conditions for a surface layer.
Japanese Patent Application Laid-open No. H08-171220 discloses optimum values for the N/Si element composition ratio and O/Si element composition ratio of the outermost surface of a photosensitive member and conditions for generating the values. However, in Japanese Patent Application Laid-open No. H08-171220, only a wavelength to be used for exposure up to 550 nm is taken into consideration. Furthermore, Japanese Patent Application Laid-open No. H08-171220 describes that a thickness of a surface layer in excess of 0.8 μm results in a reduction in sensitivity. That is, a thickness of the surface layer in excess of 0.8 μm results in a reduction in sensitivity even at an exposure wavelength of 550 nm. Therefore, light is expected to be absorbed to some extent at a wavelength of, for example, around 400 nm, ans a sufficient sensitivity may not be obtained.
That is, a first important point is that almost no exposure light at a wavelength as short as about 400 nm is absorbed in the surface region of a photosensitive member. A second important point is that the photosensitive member has a sufficient function of blocking the injection of charges from its surface. A third important point is that the photosensitive member is high in resolution so that it can take advantage of a small spot diameter and toner having a small particle size.